Angular-momentum-state distribution of sodium atoms photoassociated in a magneto-optical trap

The study of ultracold collisions between groundand excited-state atoms has received a great deal of attention @1#. Trap-loss measurements have revealed hyperfine@2# and fine-structure-changing collisions @3#, the rich detail of ultracold photoassociative spectroscopy has provided important information about long-range atom-atom interaction potentials @4#, and intensity and time-resolved studies have provided insight into the kinetics and dynamics displayed by the atoms during the collision. One important issue is the complicated behavior of the near dissociation limit ~long-range! photoassociation process that represents the common starting point of all optical cold collisions that occur in a magnetooptical trap ~MOT! @6#. Recently, novel experiments have indicated that photoexcitation at long range, followed by spontaneous emission and reexcitation, still at long range, can enhance the relative approach velocity of the collision partners @5# ~in the context of cold collisions, this excitationdecay-excitation cycle is referred to as recycling!. In a related experiment @7#, spectroscopy of ground-state molecules formed via photoassociation offered support for this picture. In this paper we present direct spectroscopic evidence that when two atoms colliding in a MOT are photoassociated, a relatively large number of angular-momentum states are populated. In a recent paper @8#, we described our use of resonantly enhanced multiphoton ionization ~REMPI! to study Na2 molecules excited in states that connect to the 3 S1/2 13 D1/223/2 asymptote. To summarize: the excitation process was a 211 REMPI scheme @8# that began when a trapping laser photon was absorbed by a colliding atom pair, thus photoassociating the atoms into a molecular state at long range, near the 3 S1/213 P3/2 asymptote @9# ~see Fig. 1!. Next, a photon from a separate tunable probe laser was absorbed. The probe laser was tuned to the red of the 3 P3/2 →3 D1/223/2 Na atomic transition at l5819 nm. Population was transferred at the inner turning point of the singly excited Hund’s case ~a! 1 Pg and 1 Pu (V50 1 component! states into the 2 Pu and 2 Pg high-lying states. Subsequently, either a trap or probe photon was absorbed and 2 Pu or 2 Pg was ionized @8#. By analyzing the structure of the ion production spectrum as a function of the probelaser detuning, we were able to extract precise molecular constants for the 2 Pu and 2 Pg states, and accurately deperturb the spectrum.